Transparent yttria ceramics and method for producing same

ABSTRACT

An article of manufacture is provided comprising a substantially transparent high density polycrystalline yttria-base body consisting essentially of yttria and from about 0.01 to 5 wt. % alumina. 
     A method for preparing the transparent yttria doped with alumina is also provided comprising admixing alumina or a precursor thereof with yttria or a precursor thereof, drying the admixed powders, calcining the admixed powders and pressing the calcined powders into a desired shape and thereafter sintering the shaped powder for about 1/4 to six hours at a temperature above the eutectic temperature at a sufficiently low oxygen atmosphere to prevent oxidative contaminants.

This is a division of application Ser. No. 814,342, filed July 11, 1977now U.S. Pat. No. 4,098,612.

BACKGROUND OF THE INVENTION

The present invention relates to transparent yttria ceramics and tomethods for the preparation thereof. More particularly, this inventionrelates to substantially transparent, high density, polycrystallineyttria doped with alumina and to methods for the preparation thereof.

Ceramics have long been considered desirable materials for use in hightemperature applications; however, ceramic materials are generallyopaque and cannot be suitably employed where light transmission isdesired.

Efforts have heretofore been made to obtain transparent ceramicsespecially for high temperature lamp envelope applications. Recently,transparent yttria-based materials have been developed. For example,U.S. Pat. No. 3,545,987 discloses a high density, yttria-basedpolycrystalline ceramic containing from about 2 to 15 mole % of an oxideselected from the group consisting of thoria, zirconia, hafnia orcombinations thereof. It has been found, however, that in such materialsanion interstitials are formed which are unstable in the low-oxygenpressure, high temperature environment of a high-pressure sodium lamp.Consequently, high-pressure sodium lamps having envelopes formed fromsuch materials have been found to be unsatisfactory since they darkenrendering the envelope opaque after a few hours of use.

U.S. Pat. No. 3,878,280 describes a method for the preparation oftransparent yttria without the use of additives by use of a vacuum hotpressing operation. This technique is not amenable, however, to theproduction of thin-wall lamp envelope tubing due to the high pressuregradients along the length of the tube. Consequently, hot pressing isnot currently considered a viable process for the formation oftransparent polycrystalline tubes.

U.S. Pat. No. 3,873,657 describes the preparation of transparent yttriaby employing beryllium compounds as a sintering aid. The berylliumcompounds employed, however, are toxic; consequently, utilization ofsuch materials in commercial manufacturing operations would be severelyhandicapped and even perhaps prevented by governmental regulations.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to overcome thedisadvantages of the prior art and to provide a substantiallytransparent yttrium oxide with excellent light transmitting properties.It is another object to provide a method for economically andnon-toxically fabricating such transparent yttria envelopes which arestable in the environment of a high-pressure sodium lamp.

These as well as other objects are accomplished by the present inventionwhich, in one embodiment, provides an article of manufacture comprisinga substantially transparent, high density, polycrystalline ytrria-basedbody consisting essentially of yttria (Y₂ O₃) and containing from about0.01 to 5 wt. % alumina (Al₂ O₃).

The present invention makes use of the increased thermodynamic stabilityof yttria (Y₂ O₃) over yttria doped with +4 valence ions, e.g. Th⁺⁴.Yttria, when doped with thoria, darkens between 1200°-1400° C. in thesodium vapor environment (an oxygen pressure of 10⁻⁷ atmospheres). Ithas now been found that if alumina (Al₂ O₃) is employed as a sinteringaid, yttria can be sintered to transparency without significantlyreducing the thermodynamic stability of yttria. The eutectic temperaturebetween the compounds Y₄ Al₂ O₉ and Y₂ O₃ is 1920° C.±10°. Above thistemperature, a liquid phase is formed which provides a means ofpromoting densification by a liquid phase sintering mechanism. Such amechanism, when employed in accordance with the process of the presentinvention, enhances densification and leads to the attainment of highdensity transparent yttria.

The transparent yttria doped with alumina in accordance with the presentinvention can be obtained by admixing alumina with yttria in aconcentration of from about 0.01 to 5% by weight and preferably fromabout 0.05 to about 0.25% by weight. The alumina can be added as Al₂ O₃powder or any precursor thereof which is convertible to alumina undercalcining or sintering conditions, e.g. Al(NO₃)₃ dissolved in methanol,precipitated Al (OH)₃ or by ball milling with Al₂ O₃ grinding media.

The yttrium oxide (Y₂ O₃) employed as a starting material in the presentinvention is preferably yttrium oxide powder of high purity.Substantially equivalent results, however, can be obtained when yttriumcompounds convertible to yttrium oxide under calcining or sinteringconditions are employed such as yttrium carbonate (Y₂ (CO₃)₃ or yttriumoxalate (Y₂ (C₂ O₄)₃, with the latter source being preferred.

The powders are admixed with each other or if a liquid media is employedthe liquid is thoroughly admixed with the powder until the powder isimpregnated therewith. Thereafter, the resulting powders are dried,preferably at about 110° C. and calcined for 1 hour at about 1000° C. inair and then screened through a 100 mesh screen. The resulting powderscan then be pressed to any desired shape such as a tube, disc or thelike at pressures ranging from about 15,000 to 50,000 psi. The pressedarticle is then pre-fired in air for 1 hour at 1,000° C. and is thentransferred to a controlled atmosphere furnace wherein it is sinteredfor about 1/4 to six hours at a temperature above the eutectictemperature, preferably at from about 2000° C. to 2400° C. and mostpreferably at about 2100° C.

Prior to sintering at a temperature above the eutectic temperature thepressed article may be sintered about 1-8 hours at a temperature belowthe eutectic temperature, preferably at about 1700° C. to 1850° C. andmost preferably at about 1850° C., and subsequent to sintering at atemperature above the eutectic temperature, the pressed article may besintered about 1 to 2 hours at a temperature at or about the eutectictemperature, at around 1875° C. to 1975° C. and most preferably at about1925° C.

The optional first stage of sintering promotes maximum densificationwith Y₄ Al₂ O₉ as a solid second phase inhibiting grain growth. It isdesirable to have as much densification as possible without pores beingentrapped within grains prior to the essential second stage ofsintering, that is, the sintering at above the eutectic temperature. Theessential second stage is the liquid phase sintering regime where theessential removal of the last few percent of porosity and grain growthoccur. The optional third stage of sintering is desirable because it isat the eutectic temperature, 1920°±10° C., that the maximum solubilityfor Al₂ O₃ in Y₂ O₃ exists. This stage minimizes the second phaseretained in the structure which may be desirable for optimized chemicaland physical properties.

The atmosphere during sintering should be at a sufficiently low oxygenpressure to prevent oxidation of the furnace elements which can giverise to contamination of the sintered body. In the case of a W meshfurnace, for example, an oxygen atmosphere less than or equal to 10⁻¹⁰atmospheres at 1900° C. and 2×10⁻¹⁰ atmospheres at 2100° C. has beenfound satisfactory. If an oxygen pressure of equal or slightly greaterthan 1×10⁻¹⁰ atmospheres is employed during the last stage of thesintering cycle wherein the temperature is maintained at or about theeutectic temperature, the resultant product is substantially colorless,transparent Y₂ O₃. If, however, the oxygen pressure is significantlyless than 10⁻¹⁰ atmospheres, the resultant product is black; but can beeasily oxidized in five hours at 1500° C. in air to obtain transparent,colorless Y₂ O₃.

The substantially transparent, high-density polycrystalline yttria dopedwith alumina obtained in accordance with the present invention providesan excellent shaped transparent ceramic for lamp envelope applications.These ceramics are thermodynamically more stable than yttria doped with+4 valence ions in a sodium lamp environment. The transparent ceramicsof the present invention can be easily pressed and sintered formingtransparent tubes and other complicated shapes. Moreover, the aluminasintering aid of the present invention is non-toxic.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following examples further illustrate the present invention. It isto be understood that these examples are for illustrative purposes onlyand neither the spirit nor scope of this invention is to be limitedthereby. Unless otherwise specified, all percentages and parts are byweight.

In the following examples density measurements were based on theArchimedes principle and performed on a Mettler analytical balance.Optical measurements on polished 1 millimeter thick disc specimens wereobtained on a Cary 14 spectrophotometer. All reported values are at awavelength of 0.6 micrometers. Total transmission of tubes was measuredwith a Hoffman Engineering Corp. 12 inch integrating sphere in thevisible wavelength range. Specular transmissions on tubes was taken witha GTE Sylvania device which measures light transmission through bothwalls along a specimen diameter.

The table below summarizes the results obtained:

                                      TABLE                                       __________________________________________________________________________                                Pressing    Density                                                                            %                                    Weight %                                                                            Source of                                                                             Source of                                                                           Sample                                                                            Pressure    (% Theo-                                                                           Total* Specular**                Sample                                                                            Al.sub.2 O.sub.3                                                                    AL.sub.2 O.sub.3                                                                      Y.sub.2 O.sub.3                                                                     Shape                                                                             (Ksi)                                                                              Sinter Cycle                                                                         retical)                                                                           Transmission                                                                         Transmission              __________________________________________________________________________    1.  0.1   Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   2100° C.-21/4 h                                                               99.3 57.5   17.0                      2.  0.69  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   2100°  C.-21/4 h                                                              99.2 59.3   5.5                       3.  0.23  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   2000°  C.-21/4 h                                                              99.5 59.9   8.3                       4.  0.028 Al.sub.2 O.sub.3 Milling                                                              Y.sub.2 (CO.sub.3).sub.3                                                            Disc                                                                              50   1900°  C.-2 h                                                                 99.7 60.6   32.0                                Balls                  2100°  C.-2 h                         5.  --    Al.sub.2 O.sub.3 Milling                                                              Y.sub.2 (CO.sub.3).sub.3                                                            Tube                                                                              27   1650°  C.-11/2 h                                                              99.8 87     3.1                                 Balls                  2100°  C.-2 h                         6.  0.1   Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   1850°  C.-5 h                                                                 99.6 71.9   34.3                                                       2100°  C.-4 h                                                          1875° C.-2 h                          7.  0.23  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   1850°  C.-5 h                                                                 99.9 80.4   27.2                                                       2100°  C.-4 h                                                          1875°  C.-2 h                         8.  0.14  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Tube                                                                              27   1850°  C.-5 h                                                                 99.6 93.7   3.1                                                        2100°  C.-4 h                                                          1925°  C.-2 h                         9.  0.05  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   1800°  C.-4 h                                                                 99.4 46     22                                                         2100°  C.-6 h                                                          1925°  C.-2 h                         10. 0.14  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   1800°  C.-4 h                                                                 99.9 71     44                                                         2100°  C.-6 h                                                          1925°  C.-2 h                         11. 0.23  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   1800°  C.-4 h                                                                 99.9 73     35                                                         2100°  C.-6 h                                                          1925°  C.-2 h                         12. 0.14  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Tube                                                                              28   1850°  C.-5 h                                                                 99.9 95.1   7                                                          2100°  C.-4 h                                                          1925°  C.-2 h                         13. 0.10  Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Tube                                                                              28   1850°  C.-4 h                                                                 99.9 95.3   5.9                                                        2100°  C.-4 h                                                          1925°  C.-1 h                         14. 0.1   Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              28   1850°  C.-4 h                                                                 99.9 66     23                                                         2100°  C.-4 h                                                          1925°  C.-1 h                         15. 0.1   Al(NO.sub.3).sub.3                                                                    Y.sub.2 (C.sub.2 O.sub.4).sub.3                                                     Disc                                                                              50   1850°  C.-5 h                                                                 99.7 65     32                                                         2100°  C.-4 h                                                          1925°  C.-2 h                         __________________________________________________________________________     *Reflection losses reduce theoretical transmission to 81.3% for disc          specimen.                                                                     **Pseudo specular transmission of tubes was along diameter and thru two       walls.                                                                   

While there has been shown and described what is at present consideredthe preferred embodiment(s) of the invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A process for preparing a substantiallytransparent, high density polycrystalline yttria-based body comprisingthe steps of:admixing alumina or a precursor thereof convertible bysintering to alumina, with yttria or a precursor thereof convertible bysintering to yttria to form a mixture, the alumina or precursor thereofbeing present in the mixture in an amount to provide between about 0.01and 5 wt. % alumina upon subsequent sintering; drying the resultingpowder; calcining the dried powder at 1000° C. in air; pressing thecalcined powder into a given shape; and sintering the shaped powder forabout 1/4 to six hours above the eutectic temperature in an atmospheresufficiently low in oxygen to prevent oxidative contamination.
 2. Aprocess as defined in claim 1 wherein the alumina precursors areAl(NO₃)₃ or Al(OH)₃.
 3. A process as defined in claim 1 wherein theyttria precursors are Y₂ (CO₃)₃ or Y₂ (C₂ O₄)₃.
 4. A process as definedin claim 1 wherein the powders are dried at 110° C.
 5. A process asdefined in claim 1 wherein the dried powder is calcined for about 1 hourat 1000° C. in air.
 6. A process as defined in claim 1 wherein sinteringis effected for 1/4 to six hours at about 2100° C.
 7. A process asdefined in claim 1 wherein sintering is effected at an oxygen pressureof about 2×10⁻¹⁰ atmospheres.
 8. A process as defined in claim 1 whereinsintering is effected at an oxygen pressure significantly lower than10⁻¹⁰ atmospheres and the resulting black product is thereafter oxidizedfor about 5 hours at 1500° C. in air to obtain a substantiallytransparent, high density polycrystalline yttria-based body.
 9. Aprocess for preparing a substantially transparent, high densitypolycrystalline yttria-based body comprising the steps of:admixingalumina or a precursor thereof convertible by sintering to alumina, withyttria or a precursor thereof convertible by sintering to yttria to forma mixture, the alumina or precursor thereof being present in the mixturein an amount to provide between about 0.01 and 5 wt. % alumina uponsubsequent sintering; drying the resulting powder; calcining the driedpowder at 1000° C. in air; pressing the calcined powder into a givenshape; and sintering the shaped powder for from about 1 to 8 hours below1920° C., about 1/4 to 6 hours above 1920° C. and about 1 to 2 hours ator about 1925° C. in an atmosphere sufficiently low in oxygen to preventoxidative contamination.
 10. A process as defined in claim 9 wherein thealumina precursors are Al(NO₃)₃ or Al(OH)₃.
 11. A process as defined inclaim 9 wherein the yttria precursors are Y₂ (CO₃)₃ or Y₂ (C₂ O₄)₃. 12.A process as defined in claim 9 wherein the powders are dried at 110° C.13. A process as defined in claim 9 wherein the dried powder is calcinedfor about 1 hour at 1000° C. in air.
 14. A process as defined in claim 9wherein sintering is effected for from about 1 to 8 hours at atemperature of from about 1700° C. to 1850° C., for from about 1/4 tosix hours at a temperature from about 2000° C. to 2400° C. and then forabout 1 to 2 hours at about 1875° C. to 1975° C.
 15. A process asdefined in claim 9 wherein sintering is effected below 1920° C. at anoxygen pressure less than or equal to 10⁻¹⁰ atmospheres, above 1920° C.at an oxygen pressure of about 2×10⁻¹⁰ atmospheres and at or about 1920°C. at an oxygen pressure of about 1×10⁻¹⁰ atmospheres.
 16. A process asdefined in claim 9 wherein sintering is effected at an oxygen pressuresignificantly lower than 10⁻¹⁰ atmospheres and the resulting blackproduct is thereafter oxidized for about 5 hours at 1500° C. in air toobtain a substantially transparent, high density polycrystallineyttria-based body.